Document 1 (JP 2011-9078 A) discloses a conventional example of lighting devices. This conventional example includes a lighting source, a power supply circuit, and a dimming controller, and the lighting source is constituted by three types of light emitting diodes. The three types of light emitting diodes have different color temperatures.
The dimming controller includes three output adjusters. Each of the three output adjusters is configured to adjust a light output of a corresponding one type of light emitting diodes.
In the conventional example disclosed in document 1, a ratio of outputs of the light emitting diodes is adjusted by use of the three output adjusters, and thereby a color temperature of light emitted from the lighting source (i.e., color of lighting) is changed in accordance with a blackbody locus.
By the way, to realize adjustment of color of lighting (light color), at least two light sources with different light colors and at least two power supplies for individual light sources are necessary. For example, it is considered that even a simplest structure of such a lighting device requires a light source for emitting light with a relatively high color temperature (high color temperature light source), a light source for emitting light with a relatively low color temperature (low color temperature light source), a power supply for the high color temperature light source, and a power supply for the low color temperature light source.
In this regard, when a lower limit of a color adjustment range (i.e., a lowest color temperature) of the lighting device is set to a color temperature of the low color temperature light source, the power supply for the high color temperature light source is required to set an output of the high color temperature light source to zero at the lower limit of the color adjustment range. In other words, the power supply for the high color temperature light source is required to be capable of continuously changing the output of the high color temperature light source in a range of 0 to 100%. Note that, 100% means the light output at rated lighting.
While, when an upper limit of the color adjustment range (i.e., a highest color temperature) of the lighting device is set to be lower than a color temperature of the high color temperature light source, the power supply for the low color temperature light source is not required to set an output of the low color temperature light source to zero at the upper limit of the color adjustment range. In other words, it is sufficient that the power supply for the low color temperature light source can continuously change the output of the low color temperature light source in a range of X (>0) to 100%. Besides, when the power supply continuously changes the output of light source in a range of 0 to 100%, as the light output comes closer to 0%, the operation of the power supply becomes unstable and the light output becomes likely to vary (flicker).
Generally, a white LED of commercially available LEDs for lighting has a color temperature in a range of 2700 K to 6500 K. This is because specifications for the chromaticity specified by American National Standards Institute (ANSI C78.377) are often used in a field of the LEDs for lighting. The aforementioned specifications define a range of the chromaticity as a range of a color temperature of 2700 K to 6500 K.
Note that, as described above, with regard to lighting devices (lighting fixtures) providing illumination light with a desired color temperature by independently adjusting light outputs of two types of light sources (white LEDs) which are a high color temperature light source and a low color temperature light source, different color adjustments ranges are required for different intended purposes.
For example, a light color close to 5000 K is required for offices, but a light color in a range of 2700 K to 4000 K is required for eating and drinking establishments such as restaurants. Therefore, to satisfy requirements for different intended purposes, the lighting device is required to be capable of adjusting a color temperature in a range of 2700 K to 5000 K. To realize lighting at the upper limit (5000 K) of the color adjustment range, it is sufficient to mix light with 6500 K from the high color temperature light source and light with 2700 K from the low color temperature light source at a predetermined ratio. In contrast, to realize lighting at the lower limit (2700 K) of the color adjustment range, it is sufficient to use only light with 2700 K from the low color temperature light source. Hence, as described above, the power supply for the high color temperature light source (6500 K) is required to be capable of continuously changing the output of the high color temperature light source in a range of 0 to 100%. Note that, when the low color temperature light source is a light source (white LED) with a color temperature lower than 2700 K, it is sufficient that the power supply for the high color temperature light source is capable of continuously changing the output of the high color temperature light source in a range of X (>0) to 100%. However, such light sources (white LED) with a color temperature lower than 2700 K are not specified by the aforementioned specifications, and are very expensive because of the low demand for them, and are much less frequently used.